The present invention relates to electrochemical devices that function by maintaining separate gaseous streams and causing reactions to occur at the surfaces of conductive layers of adjacent components.
Electrochemical devices having multiple components, such as, for example, solid oxide fuel cell (SOFC) stacks, syngas membrane reactors, oxygen generators and the like require a critical seal technology to separate gas streams (e.g., H2 and O2) and to prevent the streams from mixing with each other. Mixing of the gas streams has a variety of negative consequences, depending upon the type of device and the composition of the gaseous streams. One major problem that results from mixing of such gases is the possibility of thermal combustion of the gases and the resulting failure of the device.
One type of electrochemical device that has received, and continues to receive, significant attention is a fuel cell device. Fuel cell devices are known and used for the direct production of electricity from standard fuel materials including fossil fuels, hydrogen, and the like by converting chemical energy of a fuel directly to electrical energy. This conversion is accomplished by oxidizing the fuel without an intermediate thermal energy stage. Fuel cells typically include a porous anode, a porous cathode, and a solid or liquid electrolyte therebetween. Fuel (e.g., hydrogen) is fed to the anode where it is oxidized and electrons are released to the external circuit. Oxidant (e.g., oxygen) is fed to the cathode where it is reduced and electrons are accepted from the external circuit. The electron flow through the external circuit produces direct-current electricity. The electrolyte conducts ions between the two electrodes.
Fuel cells are classified into several types according to the electrolyte used to accommodate ion transfer during operation. Examples of electrolytes include aqueous potassium hydroxide, concentrated phosphoric acid, fused alkali carbonate, solid polymers, e.g., a solid polymer ion exchange membrane, and solid oxides, e.g., a stabilized zirconium oxide. Solid oxide fuel cell (“SOFC”) devices have attracted considerable attention as the fuel cells of the third generation following phosphoric acid fuel cells and molten carbonate fuel cells of the first and second generations, respectively. SOFC devices have an advantage in enhancing efficiency of generation of electricity, including waste heat management, with their operation at high temperatures, typically above about 650° C.
Those involved in research and development of SOFC technology consider SOFC power generation as an emerging viable alternative to the use of internal combustion engines. Contrary to internal combustion, the oxygen is transported in a SOFC device via the vacancy mechanism through a dense ceramic electrolyte, and then reacted with the hydrogen electrochemically. Because the SOFC converts the chemical energy to electrical energy without the intermediate thermal energy step, its conversion efficiency is not subject to the Carnot Limit. Compared to conventional power generation, SOFC technology offers several advantages, including, for example, substantially higher efficiency, modular construction, minimal site restriction, and much lower air pollution.
In a typical SOFC, a solid electrolyte, made of dense yttria-stabllized zirconia (YSZ) ceramic, separates a porous ceramic anode from a porous ceramic cathode. The anode typically is made of nickel/YSZ cermet, and the cathode is typically made of doped lanthanum manganite. In such a fuel cell, an example of which is shown schematically in FIG. 1, the fuel flowing to the anode reacts with oxide ions to produce electrons and water. The water is removed in the fuel flow stream. The electrons flow from the anode through an external circuit and thence to the cathode. The oxygen reacts with the electrons on the cathode surface to form oxide ions that diffuse through the electrolyte to the anode. The electrolyte is a ceramic material that is a nonconductor of electrons, ensuring that the electrons must pass through the external circuit to do useful work. However, the electrolyte permits the oxygen ions to pass through from the cathode to the anode.
When fuel is supplied to the anode and oxidant is supplied to the cathode, a useable electric current is electrochemically generated by the flow of electrons through the external circuit from the anode to the cathode. As an example, the chemical reaction for a fuel cell using hydrogen as the fuel and oxygen as the oxidant is shown in equation (1).H2+½O2→H2O  (Eq. 1)
This process occurs through two redox or separate half-reactions which occur at the electrodes as follows:
Anode ReactionH2+O2−→H2O+2e−  (Eq. 2)
Cathode Reaction½O2+2e−→O2−  (Eq. 3)
In the anode half-reaction, the hydrogen fuel is oxidized by oxygen ions from the electrolyte, thereby releasing electrons (e−) to the external circuit as shown in equation (2) and as shown schematically in FIG. 1. The oxygen ions migrate through the fuel cell electrolyte from the cathode to the anode. In the cathode half-reaction, oxygen is fed to the cathode, where it supplies the oxygen ions (O2−) to the electrolyte by accepting electrons from the external circuit. The movement of oxygen ions through the electrolyte maintains overall electrical charge balance, and the flow of electrons in the external circuit provides useful power. As alternatives to hydrogen, useful fuels for fuel cell power generation include, for example, carbon monoxide and methane.
Because each individual electrochemical cell, made of a single anode, a single electrolyte, and a single cathode, generates an open circuit voltage of about one volt, and each cell is subject to electrode activation polarization losses, electrical resistance losses, and ion mobility resistant losses which reduce its output to even lower voltages at a useful current, a fuel cell assembly comprising a plurality of fuel cell units electrically connected to each other is required to produce the desired voltage or current to generate commercially useful quantities of power.
Currently, there are two basic designs for SOFC applications: tubular and planar. With respect to planar SOFC designs, the individual electrochemical cells are typically connected together in series to form a stack. For example, planar solid oxide fuel cell stacks typically comprise a plurality of stacked cathode-electrolyte-anode-interconnect repeat units, and the fuel cell stack includes an electrical interconnect between the cathode and the anode of adjacent cells. The fuel cell assembly also includes ducts or manifolding to conduct the fuel and oxidant into and out of the stack. Channels for gas flow, either in a cross-flow or a co-flow or a counterflow configuration, are usually incorporated into the cathode, anode and/or interconnect. Planar designs are believed to potentially offer lower cost and higher power density per unit volume compared to tubular designs; however, planar designs face many challenges that must be overcome.
In addition to the challenges in materials development for electrolytes, anodes, and cathodes, planar SOFC designs require seals between each individual cell to prevent (or at least sufficiently minimize) leaking of gases from the stack as well as mixing of fuel and oxidant gases. Low fuel leak rates are required if SOFC stacks are to operate safely and economically. Furthermore, the seal needs to have long-term stability at the elevated temperatures and harsh (oxidizing, reducing and humid) environments typical of SOFCs during operation. Also, the seals should not cause corrosion or other degradation of the materials with which they are in contact (e.g., stabilized zirconia, interconnect, and electrodes). Perhaps most significantly, the seal needs to be suitably durable to acceptably perform its sealing function under repetitive thermal cycling.
A variety of features of a SOFC stack add to the difficulty of obtaining a good seal. For one, both the cell (including anode, electrolyte and cathode layers) and the interconnect, whether of ceramic or metallic material, are rigid. As a result, to achieve an effective seal, the mating surfaces between the cell and the interconnect must be flat and parallel. Nevertheless, because all of the components are rigid, even with good flatness, it is necessary to seal the surfaces in some manner to prevent leakage of the gases.
Another feature of electrochemical devices, such as SOFCs, that lends to the difficulty of obtaining a good seal relates to the fact that diverse compositions are used as the components of a SOFC device, and the diverse compositions have differing thermal expansion characteristics. In this regard, in various types of fuel cell assemblies adapted for use at high operating temperatures, a monolithic design is used in which the entire structure is made of ceramics. In other designs, individual components are rigidly and hermetically sealed using, for example, glass seals, glass-ceramic seals, cermet seals or metallic braze. While such monolithic or rigidly formed fuel cells are well equipped to prevent gas leakage, ceramics have the inherent material characteristic of low ductility and low toughness. Consequently, they are susceptible to damage by mechanical vibrations and shocks. Furthermore, and perhaps more problematic, such assemblies are extremely susceptible to thermal shocks and to thermally induced mechanical stresses due to the different thermal expansion characteristics of the components.
A wide variety of applications for which SOFC devices can be used to advantage involve intermittent power demands, and thus involve intermittent usage and nonusage, and thus repeated heating and cooling cycles. Given the variety of materials used to make a single cell, and the difficulty of selecting suitable materials that have precisely matched coefficients of thermal expansion, it is readily seen that the use of rigid seals presents significant problems. Furthermore, where the fuel cell is designed to be used at lower temperatures with a low-temperature ceramic electrolyte, some components of the fuel cell may be made of metals, which are generally less expensive to fabricate than ceramic components and have the advantage of improved ductility and fracture toughness, making them more resistant to mechanical and thermal shock damage than ceramics. However, in a fuel cell using metals for at least some components and ceramics for at least some components, rigid sealing is perhaps an even greater problem because most alloys potentially suitable for the SOFC interconnect application have much higher coefficients of thermal expansion than do ceramics, resulting in large thermal stresses and strains produced during operation of such a fuel cell. When a metal/ceramic fuel cell is heated and cooled, the dimensions of the metal components change more than the dimensions of the ceramic components, leading to thermal strains within the structure. These thermal strains produce thermal stresses that can lead to failure of the ceramic components or the rigid seals between the ceramic and metal components.
Another type of seal that has been considered for use in connection with SOFC devices is a compressive seal. In a device designed to utilize a compressive seal, a layer of inert material is placed between components of the SOFC and a compressive force is applied to the components and the material therebetween in an attempt to block leakage between the components. In comparison to rigid ceramic, glass or metallic seals, compressive seals potentially offer several advantages. Since they are not rigidly bonded to the cells, the need for matching coefficients of thermal expansion (CTE) of all stack components is reduced or eliminated. The cells and interconnects are allowed to expand and contract more freely during thermal cycling and operation, thereby reducing structural degradation during thermal cycling and routine operation. Elimination of the need for matching CTE greatly expands the list of candidate interconnect materials, whether ceramic or metallic. The compressive seals also have two unique advantages over rigid seals. One is that cells in stacks may be reusable since they are not bonded with one another. Secondly, it allows non-destructive post-service analysis
Research in the area of the compressive seals is still in its early stages and very little data is available. One group discussed the use of compressed mica in a single-cell SOFC set-up; however the effectiveness was not discussed. (Kim and Virkar, Solid Oxide Fuel Cells (SOFC VI) Proceedings of the Sixth International Symposium, edited by S. C. Sighal and M. Dokiya, The Electrochemical Society, Proceedings Volume 99-19, 830 (1999)). A recent publication discusses work relating to micas in paper form and cleaved single crystal micas as compressive seals for SOFC applications. (Simner et al. “Compressive mica seals for SOFC applications,” J. Power Sources, 102 [1-2], 310-316, (2001)). The results showed that cleaved natural mica sheets were far superior compared to mica papers. For the mica sheets, leak rates of about 0.33-0.65 sccm/cm at 800° C. and 100 psi were measured on small test coupons simulating a single interconnect/seal/cell/seal/interconnect unit. A coupon leak rate of 0.33-0.65 sccm/cm, however, is believed to translate to unacceptably high leak rates for actual SOFC stacks, in which multiple, full size components would be stacked together with the gaskets between each component.
In view of the above background, it is apparent that one important challenge in the development of SOFC assemblies and other electrochemical devices is the development of sealing technology offering suitably low leak rates. There is a continuing need for further developments in the field of seals for such electrochemical devices. The present invention addresses this need, and further provides related advantages.